The present invention relates to a method of cooling flue or process gases containing halogenous compounds in two successively located circulating fluidized bed reactors, which is a variation of that shown in co-pending U.S. application Ser. No. 07/776,403 filed Nov. 22, 1991. The method herein provides cooling in the first circulating fluidized bed reactor hot gases to a temperature of &gt;400.degree. C. Further, the method provides quickly cooling the hot gases in the second circulating fluidized bed reactor to a temperature below 250.degree. C.
The present invention relates especially to a method of cooling flue or process gases resulting from treatment of materials containing chlorine or halogenized hydrocarbons. These materials may comprise waste, biological sludge, metal concentrates, scrap metal., and slag, from, e.g., copper smelters. Further, the method is capable of minimizing environmentally harmful emissions.
The formation of chlorine-containing environmental poisons resulting from incineration of municipal waste, combustion of biological sludges, and smelting processes has been recognized as a significant problem. It has been found that polychlorinated aromatic compounds categorized as super-poisons, such as coplanar PCB, polychlorinated dioxins, and polychlorinated furans, are produced to a great extent in both municipal waste incinerator plants and industrial combustion plants for biological waste. Also, dioxins have been found in flue gases from metal smelters. These chlorine-containing compounds are super-poisonous for human beings and animals, capable of bringing about genetic damage. Further, these compounds are also carcinogenic. The environment is polluted by these toxic compounds by their attachment to flue dust.
It has been found that the lower the temperature and the shorter the retention time, the greater the amount of resulting super-poisons. Further, it has been found that combustion at 500.degree.-700.degree. C. is especially favorable for the formation of polychlorinated dioxins and furans. This has led to the conclusion that the combustion should be carried out at a higher temperature and with a longer retention time in order to decompose the super-toxins. However, it has also been suggested that the wastes could be safely destroyed by incinerating them at relatively low temperatures if the flue gases are post-combusted at such a high temperature that, for example, dioxin and furan toxins decompose.
It is, nevertheless, not always possible to maintain a temperature suitable for decomposing the super-toxins in the actual process or in post-combustion. It has additionally been found that the super-toxins are easily re-formed in the later stages of the process, and also after post-combustion.
Dioxins are formed by synthetization from chlorine compounds in the presence of coal, water and oxygen at favorable temperatures. The temperature in which flue gases exist is one example. It has been established that a large amount of the dioxins located in the fly ash are formed only after combustion in heat exchangers and the stack. The formation is caused by a means of a so-called "de novo synthesis". This is because the "de novo synthesis" of the dioxins is to a great extent dependent on the temperature, and the temperature of 250.degree. to 400.degree. C. is especially suitable for this type of synthesis. Efforts have been made, by as complete a combustion as possible, by a high combustion temperature, and by a long retention time, to produce fly ash containing as little carbon as possible, whereby the synthesis would be prevented. By separating metals from the wastes, it has also been attempted to decrease the amount of polychlorinated aromats in the flue gases.
U.S. Pat. No. 4,762,074 suggests the combustion of wastes containing prestages of toxic dioxins and furans by means of oxygenated air at a high temperature, at least 1200.degree. C., for decomposing the toxins. By using a highly excessive amount of oxygen, it is also attempted to maintain the amount of the nitrogen oxides, which increase at high temperatures, at an acceptable level.
Swedish patent specification 453,777 discloses a method of combusting solid waste in a reactor in a bubbling fluidized bed at a temperature below 900.degree. C. In the reactor, above the actual fluidized bed, the temperature of the flue gases is raised to at least 950.degree. C. by adding secondary air and insulating the reactor walls. The temperature of the flue gases is maintained at the level above 950.degree. C. for as long as required for the hydrocarbons and dioxins to become decomposed, by conducting the flue gases through an uncooled gas duct. The gases are cooled prior to the heat exchangers by injecting air or flue gases into the gases.
U.S. Pat. No. 4,794,871 discloses a method, comprising two or three stages, in which waste is first treated in a rotary drum at a temperature of at most 500.degree. C. and the solid waste formed thereby is then treated at a temperature of at least 500.degree. C., preferably at 500 to 1000.degree. C., for destroying the toxic substances. The flue gases from both stages are combined and combusted at such a high temperature that the toxic compounds of the gases are completely decomposed.
Swedish patent application 8406090-4 discloses a method in which wastes harmful to the environment and containing chlorinated hydrocarbons are incinerated substoichiometrically at a temperature of at least 1200.degree. C. The combustion is effected as plasma incineration. According to the patent application, re-formation of toxic compounds is prevented by bringing the flue gases that have been cooled to a temperature of 350.degree. to 700.degree. C., into contact with lime for separating chlorine from the gases. Chlorine separation is preferably effected by conducting the gases through a vertical reactor filled with lime. At low temperatures, however, polychlorinated aromats are produced quickly before the chlorine has become bound to lime. Thus, the method does not lead to a desirable result.
U.S. Pat. No. 4,938,366, among others, suggests decomposing the toxic polyhalogens in a two-stage catalytic treatment. In the method the gases are first directed through an oxidizing catalytic cracking stage and then directed through a catalytic post-combustion stage.
An object of the present invention is to provide a simple method of minimizing the emissions of polyhalogenated hydrocarbon compounds in plants forming hot process or flue gases.
Characteristic features of the method according to the invention are: first--the process or flue gases, the temperature of which is &gt;700.degree. C., are introduced as fluidizing gas to the first circulating fluidized bed reactor and cooled therein to 400.degree.-700.degree. C.; second--the retention time of the gases in the first circulating fluidized bed reactor is about 1-10 seconds; and third--in the first circulating fluidized bed reactor the gases are brought into contact with a catalytic material capable of decomposing polyhalogen compounds.
Preferably the retention time of the gases in the first circulating fluidized bed reactor at a temperature of 400.degree.-700.degree. C. is 2-5 seconds. In this amount of time the catalytic material effects a complete decomposition of the dioxins and furans contained in the gas.
Preferably the catalyst is a metal powder. The metal powder may be Zn, Cu, Ni, Cr, Fe, Al, Pb, other metal or alloy. Also, the catalyst may be a metal oxide, such as ZnO, NiO, Cr.sub.2 O.sub.3, FeO, Fe.sub.3 O.sub.4, PbO, CaO, MgO, other metal oxides or mixtures thereof. Further, the catalyst may comprise a metal salt, such as Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, or CaCO.sub.3. Preferably, the catalytic material consists of a catalytically active metal, metal oxide or metal salt dust, which is entrained with the process gas and allowed to form at least partly the bed material of the circulating fluidized bed reactor. The circulating fluidized bed reactor can be located, for example, after the shaft furnace of a scrap copper smelter. The flue gases from the furnace contain the contaminants contained in the scrap material, for example ZnO, PbO, SnO etc. These contaminants are separated in the dust separator of the circulating fluidized bed reactor to remain as bed material and act thus as catalysts to decompose organic chlorine compounds.
In a circulating fluidized bed reactor the hot gases conducted therein as fluidizing gas are very quickly cooled when they are mixed with the circulating bed particles colder than the gas. Usually the rate of cooling in the mixing chamber is &gt;500.degree. C./s, preferably more than 1000.degree. C./s.
According to an embodiment of the invention, the cooling installation has been connected to the radiation chamber of a waste heat boiler of a metal smelter. The gases are exhausted at a temperature of about 700.degree.-1200.degree. C. from the radiation chamber and are then directed as fluidizing gas to the first circulating fluidized bed reactor, wherein the gases are cooled to 400.degree.-700.degree. C. The bed material is at least partly formed by metal oxide, e.g., zinc oxide, which acts as a catalyst in the decomposing reaction of the dioxins and furans. The retention time of the gases in the reactor is preferably 2-5 seconds. If necessary, ash-containing bed material can be removed from the circulating fluidized bed reactor. New catalyst-containing bed material is introduced into the reactor either by adding some of it, as needed, or by allowing at least some of the metal oxide dust contained in the flue gases to remain as bed material.
From the first circulating fluidized bed reactor the gas, from which the catalyst has been separated, is introduced into the second circulating fluidized bed reactor, in which the final cooling of the gas is effected. When the gases are exhausted from the first reactor, they are in a temperature range of about 400.degree.-700.degree. C. These gases are rapidly cooled in the second reactor past the temperature area of 250.degree.-400.degree. C. This temperature area is where the "de novo" synthesis of dioxins and furans takes place, and this is desirable to reduce the temperature to below 250.degree. C. as quickly as possible. The rate of cooling is &gt;500.degree. C./s, preferably &gt;1000.degree. C./s. Thus, the gases are cooled almost immediately to a temperature of below 250.degree. C. upon entering the mixing chamber of the second reactor. In the second circulating fluidized bed reactor the gases can be cooled to 180.degree. C., or even below that, if needed, after which the gases can be conducted to a bag filter, or other similar member, for final cleaning.
By cooling the gases rapidly past the temperature area most suitable for dioxin synthesis, or similar synthesis, at 250.degree.-600.degree. C., the formation of these polyhalogenous aromatic compounds is minimized. Further, substances that either react with the chlorine, fluorine and sulphur compounds, or absorb these compounds, can be added to the circulating bed material of either the first or the second reactor to remove these compounds from the gases. At the same time a considerable part of the heavy metals and their compounds that are contained in the flue gases and that melt and vaporize at a low temperature, such as Hg, As, Zn, Cd, Pb and Sn, can be separated from the gases by condensating them into the circulating bed material in the cooling reactors. Particles can be removed from the circulation of the cooling reactor either continuously or intermittently for removing harmful or recoverable substances from the circulation. New particles are accordingly added to maintain the circulating material needed for the gas cooling process.